There is a continuing drive in radiotelephone receiver design to improve the linearity characteristics, the power consumption and the noise figure of the receiver circuitry whilst achieving a suitable level of receiver gain. Image reject mixer circuits are commonly used circuit blocks of such receivers. Image reject mixer circuits in which RF input signals are arranged to be fed into first and second parallel paths, associated with in-phase and quadrature local oscillator signals respectively, and subsequently combined are generally preferred to mixer circuits which have a filter to reject image frequency signals. This preference stems from the fact that their noise figure is comparable to that obtained when an ideal image reject filter is used, which of course is not possible, and they tend to take up less chip area and/or involve fewer discrete components than mixer circuits having image reject filters. Whatever type of image reject mixer circuit is used in a radio receiver, its parameters determine the main characteristics of the receiver.